Apparatuses for generating carrier gas streams containing controlled partial pressures of one or more target gases are used in a variety of industries. For example, bubblers are widely used in the semiconductor industry for delivery of gases to processing equipment. As referred to in U.S. Pat. No. 5,078,922 the disclosure of which is hereby incorporated by reference, a prior bubbler utilizes a carrier gas inlet to which is attached a horizontally oriented sparger tube with a plurality of mechanically formed exit holes through which carrier gas streams enter the liquid chemical. The streams of carrier gas provided by the plurality of small exit holes in the sparger tube bubble up through the liquid chemical. Some of the liquid chemical is vaporized by the carrier gas to form a chemical vapor. Carrier gas and chemical vapors exit the bubbler chamber through a vapor outlet tube.
The inclusion of a sparger on the carrier gas inlet tube improves mass transfer between the liquid and vapor phases in the bubbler chamber by decreasing bubble diameter relative to earlier inlet designs with a single gas inlet hole. Smaller bubbles rise through the liquid chemical more slowly, thus allowing greater time for diffusion of chemical vapors into the bubbles. Additionally, the gas-liquid interfacial area per volumetric flow rate of the carrier gas increases as the bubble diameter decreases. Diffusive flux is directly proportional to the interfacial area, and bubble-liquid interfacial area increases as the inverse of the bubble diameter squared. Thus, for a given volumetric flow rate of carrier gas, mass transfer of vaporized liquid chemical into the carrier gas stream increases as the bubble diameter decreases. However, the minimum bubble diameter achievable with a sparger is limited by the size and number of holes than can be produced by mechanical means in the sparger tube. Alternative and more effective means for generating more numerous streams of smaller bubbles while maintaining high carrier gas flow rates are desirable.
In general, the flow of chemical vapor that may be supplied by a bubbler is governed by the following equation:                               Q          Chemical                =                              Q                          Carrier              ⁢                                                           ⁢              Gas                                ⁡                      (                                          P                vapor                                                              P                  Head                                -                                  P                  vapor                                                      )                                              (        1        )            
where QChemical is the volumetric flow rate of the chemical (standard cm3 minute−1 or sccm), QCarrier Gas is the volumetric flow rate of the carrier gas (sccm), Pvapor is the partial pressure of the chemical at the bubbler temperature, and PHead is the gauge pressure in the bubbler chamber. The rate of chemical vapor production, QChemical may be increased by increasing the carrier gas flow rate, QCarrier Gas, or by increasing the temperature to increase Pvapor. Because PHead is also a function of temperature, increases in QChemical are most often attained by increasing QCarrier Gas. However, prior art bubblers generally have a maximum carrier gas flow rate in the range of approximately 1 to 5 standard liter per minute (slm) depending on the bubbler temperature. Increasing carrier gas flow rates result in splashing and entrainment of liquid droplets into the carrier gas stream.
Increasing the carrier gas flow rate in a prior art bubbler system also generally results in lower gas-phase concentration of the vaporized chemical in the carrier gas stream exiting the bubbler due to increased bubble diameters for a given sparger tube outlet hole size and the resulting decrease in carrier gas residence time in the fluid and cross sectional area for diffusive flux and 2) increased churning of the liquid chemical leading to generation of suspended liquid-phase droplets that may become entrained in the carrier gas flow exiting the bubbler chamber through the vapor outlet tube. Subsequent deposition of these droplets downstream of the bubbler can lead to problems of contamination or poor control of the gas-phase concentration of the chemical vapor.
Alternative methods for generating chemical vapors at high flow rates have been previously developed. These methods include direct liquid injector (DLI) devices. In a DLI, pressurized liquid is delivered to a heated unit that causes rapid volatilization into the flow stream. The concentration of the resulting vapors in the carrier gas stream is a function of the flow rate of the carrier gas through the unit and the liquid into the heated region. Because relatively small variations in the liquid delivery rate of the liquid-mass flow controller may induce large changes in the concentration of delivered vapor in DLI systems, the exact mass flow rate of the chemical is very difficult to control, and often an external closed-loop control system is used in conjunction with the liquid-mass flow controller. In bubbler systems, however, the carrier gas can be easily controlled to achieve an accurate mass flow rate of the chemical since the vapor concentration is fixed by temperature. Variations in the gas flow rate through a bubbler have decidedly smaller impacts on the resultant partial pressure of the chemical vapors in the carrier gas stream. Accordingly, there is a need for an improved bubbler design that would facilitate higher carrier gas throughput rates.